The present invention relates generally to a radio wave absorbent used to prevent TV, and radio interferences (ghosts, etc.) and to constitute an anechoic chamber, and more particularly to a radio wave absorbent using an Ni--Cu--Zn base ferrite.
Various interference problems are found to arise in the reception of radio waves used for public broadcasting such as television, and radio broadcasting as well as for mobile communications. Among others, interferences due to radio waves reflected at buildings, bridges, and mobile bodies such as vehicles (especially high-speed mobile vehicles such as those on the Shinkansen Line)--which have been introduced through urbanization and with the advancement of traffic networks--appear in the form of ghosts in the case of televisions and in the form of multipath errors in the form of FM radios, offering a leading cause of making the utilization of media worse or doing damage to mobile communications.
Radar used for airport controls, etc., on the other hand, is now subject to structural limitations. For instance, search waves are reflected at radar structures, causing interferences, and to avoid such situations there may be no choice but to restrict the zone to be searched.
To prevent interferences caused by such reflected waves, a radio wave absorbent is used. The radio wave absorbent is also used as a material for constituting a so-called anechoic chamber which enables a wave-free environment to be achieved within a certain building.
Among such radio wave absorbents known so far in the art, there is an Ni--Cu--Zn base ferrite magnetic material represented by MFe.sub.2 O.sub.4 where M is Mn, Ni, Cu, Zn, Mg or Co, as typically disclosed in JP-A 52-19046. A radio wave absorbent having such composition has a matching thickness of about 6.4 to 7.4 mm, and is found to show an attenuation of 20 dB or more at a frequency band of 50 to 400 MHz.
In this connection, when a radio wave absorbent is used for exterior or interior finish work for a certain structure, it must be processed in conformity with the contour, standard, etc. of the structure. For instance, when radio wave absorbent panels are used on the outer wall of a certain building, they may be spaced away from each other or may not be located at predetermined positions unless they are processed with a constant dimensional accuracy. However, conventional radio wave absorbents are susceptible to chipping or cracking upon processing or execution due to their low mechanical impact resistance. This, in turn, makes yields worse, resulting in cost increases.
A radio wave absorbent offers a problem in connection with matching thickness when used as an exterior material, etc. for a huge structure such as a building or bridge. That is, when a radio wave absorbent is used as the exterior material for such a structure, the thickness of the radio wave absorbent at which the necessary performance is obtained is determined by the material of which the radio wave absorbent is constructed. It is thus required to use a radio wave absorbent material having a smaller matching thickness so as to reduce the amount of the radio wave absorbent used. Especially in the case of an exterior material for a building or bridge, a large amount of radio wave absorbents is needed. For instance, a matching thickness decrease of 0.1 mm means that some considerable amount of radio wave absorbents can be saved. As an example, a matching thickness increase of 0.1 mm is tantamount to a weight increase of 5 tons in the case of an outer wall of 10,000 m.sup.2. Especially when weight is a predominant factor in determining the strength and performance of a structure like a bridge, the weight reduction is a great problem to be solved; even a matching thickness decrease or increase of 0.1 mm or less is of significant importance. For this reason, it is required to achieve a matching thickness decrease of 0.1 mm even at the lowest.
When a radio wave absorbent is used as an exterior material for a structure, it is required to have a high Curie point. More specifically, when a radio wave absorbent is used as an exterior material for a building or the like, it is preferable to have a Curie point of 80.degree. C. or higher in view of an ambient temperature as well as heat from direct sunlight. Another consideration in this regard is the influence of heat generated due to friction.
It is therefore an object of the present invention to provide a radio wave absorbent which is particularly suitable for use as an exterior material for a building or the like, has a smaller matching thickness and is excellent in chipping resistance so that it is unlikely to fail upon processing or execution, and has a practically high-enough Curie point.